1. Field of the Invention
The, present invention relates to cellular telephone systems. More specifically, the present invention relates to a novel and improved system for providing more efficient soft handoffs in a code division multiple access (xe2x80x9cCDMAxe2x80x9d) cellular telephone system to accommodate uninterrupted voice and high data rate transmission.
2. Description of Related Art
The next generation of wireless networks will provide multiple services requiring high data rate transmission and uninterrupted connections. This next generation is often referred to as the xe2x80x9cthird generationxe2x80x9d of CDMA wireless systems. The range of services include text paging, two-way radio connections, internet connectivity using microbrowsers, two-way wireless e-mail capability and wireless modem functionality. The CDMA cellular telephone system offers the capability to provide reliable radio links between a wireless communications device such as a mobile station (xe2x80x9cMSxe2x80x9d) and a base station (xe2x80x9cBSxe2x80x9d) with a much higher data capacity than conventional networks that only support voice service. As an example, in the third generation CDMA wireless systems, radio links supporting high rate (up to 2 Mbps) data transmissions will be established between the MS and the BS to provide multimedia services such as Internet access.
One particularly important feature of CDMA systems for effective third generation wireless communication is the soft handoff, which allows the MS to move smoothly from the coverage of one cell to another without interruption. The soft handoff is accomplished by establishing simultaneous communications between the MS and multiple base stations. A soft handoff is illustrated in FIG. 1. A MS 10 passes to the edge of the coverage area 12a of a serving BS 12.
While the MS 10 is within a serving BS coverage area 12a and a receiving BS 14 coverage area 14a, both base stations 12, 14 simultaneously communicate with the MS 10. As the MS 10 passes further into the coverage area 14a of the receiving BS 14, the server BS 12 stops communicating with the MS 10. In this manner, there is uninterrupted communication for the user of the MS 10 as he or she passes from the serving cell to the receiving cell. An efficient soft handoff algorithm plays an important role in maintaining the link quality as well as conserving the capacity-related network resources. As the demand to support high rate data services increases, the need to improve the efficiency of the handoff algorithm becomes more critical.
For a third generation system based on CDMA technologies, a highly efficient handoff algorithm is essential to successfully provide the infrastructure to support the new range of services. A conventional protocol for soft handoffs in a CDMA system has been adopted by the Telecommunications Industry Association in the industry standards IS-95, IS-95A or IS-95B (collectively xe2x80x9cIS-95 A/Bxe2x80x9d) for implementing a CDMA cellular system. Under the IS-95 A/B standard, a MS communicates with one or more base stations dispersed in a geographic region. Each BS continuously transmits a pilot channel signal having the same spreading code but with a different code phase offset. Phase offset allows the pilot signals to be distinguished from one another, which in turn allows the base stations to be distinguished. Hereinafter, a pilot signal of a BS will be simply referred to as a pilot. The MS monitors the pilots and measures the received energy of the pilots.
The IS-95 A/B standards define a number of states and channels for communication between the MS and the BS. For example, in the Mobile Station Control on the Traffic State, the BS communicates with the MS over a Forward Traffic Channel, and the MS communicates with the BS over a Reverse Traffic Channel. During a call, the MS must constantly monitor and maintain four sets of pilots Collectively referred to as the Active Set, the Candidate Set, the Neighbor Set, and the Remaining Set. The Active Set comprises pilots associated with the Forward Traffic Channel assigned to the MS. The Candidate Set comprises pilots that are not currently in the Active Set but have been received by a particular MS with sufficient strength to indicate that the associated Forward Traffic Channel could be successfully demodulated. The Neighbor Set comprises pilots that not currently in the Active Set or Candidate Set but are likely candidates for handoff. The Remaining Set comprises all possible pilots in the current system on the current CDMA frequency assignment, excluding the pilots in the Neighbor Set, the Candidate Set, and the Active Set.
The MS constantly searches the Pilot Channel of neighboring base stations for a pilot that is sufficiently stronger than a threshold value. As the MS moves from the region covered by one BS to another, the MS promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the BS or base stations of the promotion via a Pilot Strength Measurement Message (xe2x80x9cPSMMxe2x80x9d). The BS determines an Active Set according to the PSMM, and notifies the MS of the new Active Set via a Handoff Direction Message. When the MS commences communication with a new BS in the new Active Set before terminating communications with the old BS, a xe2x80x9csoft handoffxe2x80x9d has occurred.
In IS-95 A/B compliant CDMA systems, each BS is identified by the pseudo-random (xe2x80x9cPNxe2x80x9d) offset of its pilot channel signal. The details of the PN offset identification procedures in IS-95 A/B are well known to those of ordinary skill in the art and are therefore not discussed further herein. The MS categorizes all pilots into different sets based on the pilot""s likelihood to be used as a candidate for handoff.
The value T_ADD consists of the pilot strength threshold specified by the BS (IS-95 A) or dynamically determined at the MS (IS-95 B), above which the pilot is considered sufficiently strong to be added to the Active Set. The value T_DROP reflects the pilot strength threshold below which the pilot is considered sufficiently weak to be removed from the Active Set. The PSMM is sent from the MS to the BS to report the strength of all pilots in the Active and Candidate Sets. In response to the PSMM, an Extended Handoff Direction Message (xe2x80x9cEHDMxe2x80x9d) which includes an updated Active Set is sent from the BS to the MS.
The IS-95 A/B compliant MS typically has a searcher unit that continuously measures the pilots in various sets and reports to the BS the pilots that are sufficiently strong for an addition to the Active Set, and the pilots that are relatively weak to be removed from the Active Set. Pilots in the Neighbor Set are of particular importance, and normally they are more frequently measured than pilots in the Remaining Set.
The procedure of adding a pilot from the Neighbor Set to the Active Set in the IS-95 A/B soft handoff algorithm is briefly described as follows:
1. Each BS has a stored Neighbor List (xe2x80x9cNLxe2x80x9d) in terms of the PN offsets and configuration information of the neighboring cells. The MS receives a Neighbor List Update Message (xe2x80x9cNLUMxe2x80x9d) containing the NL from the BS and places the corresponding pilots into the Neighbor Set.
2. The MS is required to perform continuous measurement of the pilot channel strength of every pilot in the Neighbor Set using its searcher unit.
3. The MS compares the measured pilot strength with the T_ADD. Those neighbor pilots whose strengths are above T_ADD are placed in the Candidate Set and the PSMM is sent to the BS.
4. Based on the content of the PSMM and the availability of the network resources, the BS sends an EHDM to the MS indicating a new Active Set.
A similar reporting procedure is followed when the MS needs to delete a pilot from its Active Set. In this case, the strength of a pilot in the Active Set is compared with the threshold T_DROP and a timer T_TDROP is activated whenever the pilot strength decreases below T_DROP. Upon the expiration of T_TDROP, a PSMM is sent to the BS and the BS usually responds with an EHDM indicating a reduced Active Set.
According to the IS-95 A/B standard, when the MS receives a NLUM, it increments a counter corresponding to each pilot in the Neighbor Set and adds to the Neighbor Set each pilot named in the NLUM, if such pilot is not already a pilot of the Candidate Set or Neighbor Set. If the MS can store in the Neighbor Set only xe2x80x9ckxe2x80x9d additional pilots and more than xe2x80x9ckxe2x80x9d new pilots were sent in the NLUM, the MS 10 stores the first xe2x80x9ckxe2x80x9d new pilots listed in the message. More details regarding the maintenance of the Neighbor Set are found in the IS-95 A/B standards.
A more complete description of compatibility requirements for handoffs is found in the IS-95 A/B standards, and such information is incorporated herein by reference. Under the IS-95A standard, the pilot strength threshold is specified by the BS as part of an overhead information operation wherein the BS sends system parameter data to the MS periodically. As part of the overhead signal, a System Parameters Message from the BS to the MS includes the pilot detection threshold T_ADD. More details regarding the overhead information are found in Section 6.6.2.2 xe2x80x9cResponse to Overhead Information Operationxe2x80x9d of IS-95A, which is incorporated herein by reference. In the IS-95 B standard, the pilot strength threshold is dynamically determined at the MS. The relevant portions of IS-95 B which further discuss how the pilot strength threshold is dynamically determined are incorporated herein by reference.
The present soft hand-off algorithm does not provide soft-handoffs sufficiently efficient for third generation wireless services, however. Typically, the Neighbor List sent by the BS is a static list that is determined at the time the network system is deployed. It contains a list of the neighbor pilots that could be possibly xe2x80x9cseenxe2x80x9d within the cell coverage. In the IS-95 A standard, the minimum supported Neighbor Set size is 20 pilots, as represented by the N8m constant in Appendix D of IS-95 A. In the IS-95 B standard, the minimum supported size of the Neighbor Set is 40. It is not uncommon for the BS to send a NL with the maximum number of neighbor pilots just to be on the safe side, especially in a poorly optimized network.
Since the Neighbor Set pilots are the most likely handoff candidates, the frequency and the accuracy of the Neighbor Set pilot measurements greatly affects the handoff performance. However, the MS typically only has limited signal processing capabilities due to its power, size and cost constraints. Passing a large NL to the MS means that the MS has to distribute its limited searcher power among many pilots which may (and typically does) result in the poorer estimation of every pilot. A reduced sampling rate for each pilot inhibits the MS""s ability to estimate the strength of each pilot accurately. Link failures occur more frequently due to missed detections of fast time-varying pilots.
In the current handoff procedure, the BS makes the handoff decision based only on the MS""s measurement reports of the forward link pilot channel strength (F-PICH). A handoff procedure is usually triggered by the PSMM sent from the MS when it sees a pilot with sufficiently strong or weak strength. Although there exists a mechanism by which the BS can autonomously order the MS to send a PSMM, the BS solely relies on the MS""s ability to estimate and report the strength of its surrounding pilots to make handoff decisions.
There are at least three factors that could lead to the degradation in performance in the handoff algorithm. First, compared with the BS, the MS""s processing power is more restrictive which limits its pilot searching ability, especially when it has to search a large number of pilots as a result of un-optimized NL. Second, the time spent in sending the PSMM and waiting for an EHDM can sometimes be too long for the MS to react to rapid variations of the radio link conditions. Third, the forward link quality only approximately reflects the reverse link quality. Therefore, a handoff decision based only on the F-PICH measurements may not avoid failures caused by the reverse link degradation.
Attempts have been made to improve the soft-handoffs in a CDMA system. For example, U.S. Pat. No. 5,920,550, to William D. Willey, (xe2x80x9c""550 patentxe2x80x9d), assigned to the assignee of the present invention and whose contents are incorporated herein by reference, teaches providing at least one of the current measured pilot signal strengths to the BS in each access probe. The system then specifies the base stations for soft handoff according to the current measured pilot signal strengths. The ""550 patent, while improving a soft-handoff operation by reporting the current pilot strength in access probes subsequent to the System Access State, nevertheless fails to further provide the necessary efficiency and uninterrupted service that is necessary for third generation wireless communications. The ""550 patent teaches receiving a large-sized NL from the BS as indicated in the IS-95 A/B standard. Although a current pilot strength will be reported in subsequent access probes, the ""550 patent teaches distributing the MS limited searcher power among many pilots which may (and typically does) result in the poorer estimation of every pilot.
Another attempt to improve the soft handoff in a CDMA system is found in U.S. Pat. No. 5,854,785, to William D. Willey (xe2x80x9c785 patentxe2x80x9d ), assigned to the assignee of the present invention and which contents are incorporated herein by reference. The ""785 patent teaches improving the soft handoff by measuring the neighbor pilot strengths while in the System Access Mode and providing the identities of the base stations corresponding to the measured pilot strengths to the system in the initial access probe. The system uses the neighboring BS identities and pilot signal strengths to determine which neighboring BS has sufficient measured pilot strength so that an associated Paging Channel may be successfully demodulated. Thus, during a soft handoff, the MS demodulates the paging channel from at least one neighboring pilot as well as the MS""s currently active pilot.
The ""785 patent still fails to provide the necessary capability for third generation wireless applications. Although a paging channel message will be demodulated from a neighboring BS with a sufficient pilot strength, the ""785 patent teaches distributing the MS limited searcher power among many pilots which may (and typically does) result in the poorer estimation of every pilot.
The cdma 2000 family of standards were established to accommodate the third generation wireless communication systems. The family of standards include: IS-2000-1; IS-2000-2; IS-2000-3; IS-2000-4; IS-2000-5; and IS-2000-6. Each of these standards specifies a portion of a spread spectrum radio interface that uses CDMA technology and/or analog dual-mode technology for mobile stations and base stations. The cdma2000 standards are backward compatible with IS-95 B.
Many new features have been introduced in the cdma2000 proposal in an effort to further increase the system capacity. One of the features is the reverse link pilot channel transmitted by each MS in the traffic state. The reverse pilot channel is an unmodulated spread spectrum signal which is used to assist the BS in detecting a MS transmission. When in the traffic state, the MS communicates with the BS using the forward and reverse traffic channels. Adding the reverse pilot channel enables coherent detection of the mobile transmit signal at the BS and allows the system to implement fast forward link power control. The fast forward link power control is implemented by the MS inserting a reverse power control sub-channel on the reverse pilot channel.
The IS-2000-2 portion of the cdma2000 family of standards defines the physical layer standard for cdma2000 spread spectrum systems. In this specification, the structure of the reverse pilot channel includes a power control group consisting of the reverse pilot channel signal contained in the first 1152xc3x97N PN chips, and the reverse power control sub-channel in the following 384xc3x97N PN chips, where N is the spreading rate number. For example, N=1 for spreading rate 1 and N=3 for spreading rate 3. More details regarding the reverse power control sub-channel are found in Section 2.1.3.1.10 (and subsections) of IS-2000-2 which is incorporated herein by reference.
Although the cdma2000 family of standards provide some benefits through increasing system capacity and by providing reverse pilot strength measurements, the standards still fail to provide sufficiently efficient soft handoffs with uninterrupted data transmission. Therefore, the IS-95 A/B standards and the cdma2000 family of standards do not adequately address providing uninterrupted voice and data transmission during a soft handoff between a MS and a BS.
What is needed in the art is a CDMA system which improves the efficiency and uninterrupted connection between a MS and a BS during a soft handoff. The invention disclosed and claimed herein improves the existing soft handoff algorithm by using the reverse pilot strength measurements at the BS as defined in the cdma2000 standard IS-2000-2 to optimize the neighbor list. The optimization leads to enhanced handoff efficiency measured by the MS""s speed to handoff and its usage of network resources.
The field data collected during field trials for CDMA markets indicates that in a properly optimized network that the number of pilots with sufficient strength (Ec/Io greater than xe2x88x9214 dB in most cases) xe2x80x9cseenxe2x80x9d by a particular MS should be no more than 3. Even in a poorly optimized network subject to pilot pollution, the number of competing pilots at any given time and location is at most 6. Thus, the present algorithm unnecessarily requires the MS to frequently monitor more neighboring base stations than is necessary or efficient.
To address the deficiencies described above, the present invention comprises a method of handing off a wireless communication device between at least one serving cell and a receiving neighboring cell in a wireless communication system. The method comprises:
(1) establishing a call between the wireless communication device and the at least one serving cell, the at least one serving cell having a list of at least one neighboring cell which neighbors the respective serving cell;
(2) monitoring the reverse channel signal strength received from the wireless communication device from at least one neighboring cell;
(3) transmitting to a BS controller the wireless communication device signal strength detected at each neighboring cell;
(4) compiling at the BS controller a list of effective neighboring cells from the at least one neighboring cell based on the monitored wireless communication device signal strength;
(5) transmitting the list of effective neighboring cells to each at least one serving cell;
(6) periodically sending a neighboring cell list update message to the wireless communication device, the neighboring cell list update message including the effective neighboring cell list;
(7) storing the effective neighboring cell list as a neighbor set in the wireless communication device;
(8) performing forward channel signal strength searching of the neighbor set in the wireless communication device after storing the effective neighbor list as the neighbor set; and
(9) monitoring the signals from the effective neighboring cells to accomplish a handoff between the at least one service cell and the receiving neighboring cell.
Although the above steps are numbered, they do not need to be practiced the above order. The invention also includes a wireless communication system comprising:
a wireless communication device which communicates with a serving cell;
at least one cell neighboring the serving cell which monitors a reverse channel signal strength from the wireless communication device; and
a BS controller which compiles a list of effective neighboring cells from the at least one cell neighboring the serving cell based on the monitored wireless communication device signal, the BS controller communicating the list of effective neighboring cells to the serving cell, wherein the serving cell periodically sends a neighbor list update message containing the list of effective neighboring cells to the wireless communication device and the wireless communication device stores the list of effective neighboring cells as the neighbor set and performs forward channel searches on the updated neighbor set.
One of ordinary skill in the art will understand that the communication system requires other infrastructure equipment which is not shown, such as equipment for switching, call routing, and the like.